Cyclic resistance of saturated silt under wave-induced non-proportional loading

Abstract

The significance and complexity of cyclic behavior of saturated silt under non-proportional loading are still of great concern in practice, due to the highly inherent anisotropy of soil fabric. This paper presents an experimental investigation conducted in an automated hollow cylinder apparatus into the undrained anisotropy and cyclic resistance of a saturated marine silt under wave-induced non-proportional loading, which is replicated by simultaneous changeover in magnitudes and directions of the principal stress. Special attention is paid to the influence of cyclic shear mode and stress path, characterized by the cyclic stress ratio (CSR) and the ratio (δ) of the amplitude of deviatoric stress components (i.e. deviator stress and torsional shear stress), on the pore pressure response and deformation characteristics. The cyclic stress path is not a negligible factor for the cyclic resistance of saturated silt in the non-proportional loading, but had significant impact. The CSR can not properly characterize the resistance of silt to cyclic shearing, involving continuous rotation of the principal stress. This observation is considered to be directly associated with the soil fabric anisotropy. Remarkably, for all loading patterns investigated, the pore pressure ratio ru is uniquely correlated with the generalized shear strain γg, rather than to the CSR. Consequently, a closed-form expression is proposed to relate γg and ru, which can be used to predict the onset of failure as well as the pore pressure at a specific strain. By using the equivalent cyclic stress ratio (ESR) to characterize the cyclic undrained resistance of saturated marine silt (in terms of the number of cycles required to failure, NL), a virtually unique correlation between ESR and NL can be established for all the non-proportional loading investigated.